Charles Saatchi has told a court he was "utterly bereft" that a private email he sent to ex-wife Nigella Lawson referring to her being "off her head" on drugs has been made public.

The email he wrote to the TV chef was read in full to jurors but, when asked if he believed allegations that Lawson was a habitual cocaine user, the art dealer said: "I do not know."

"If you ask me whether I actually knew whether Nigella ever took drugs, the answer is no," Saatchi added as he gave evidence at Isleworth crown court in west London, where his two former personal assistants are on trial accused of fraud.

Sisters Elisabetta and Francesca Grillo are accused of using credit cards loaned to them by the celebrity couple to spend more than £685,000 on themselves.

Saatchi listened as the email he sent to Lawson on 10 October was read out to jurors. In it he referred to his ex-wife as "Higella" at one point.

He wrote: "Nigella, I was sent these by a newspaper and I can only laugh at your sorry depravity. Of course now the Grillos will get off on the basis that you … were so off your head on drugs that you allowed the sisters to spend whatever they liked and yes I believe every word the Grillos have said, who after all only stole money."

The email went on: "But I'm sure it was all great fun and now everything is perfect – bravo, you have become a celebrity hostess on a global TV game show. And you got the pass you desired, free to heartily enjoy all the drugs you want, forever. Classy."

Questioned about the email by prosecutor Jane Carpenter, Saatchi said: "I'm utterly bereft that this private email to Nigella has come back to haunt us both. That's all I have to say.

"The stories that the Grillos were parading was that Nigella had a severe cocaine habit that stretched back a very long time for the entirety of our marriage.

"What I was speculating here was that the Grillos would use this as a defence."

Saatchi said Lawson was "very cross" with him when he told her he had confronted their personal assistants about their alleged misuse of his company credit card.

The multimillionaire, who went through a high-profile divorce from the food writer earlier this year, said he thought the Grillo sisters had been "naughty" but he wanted to put the matter behind them, while Lawson wanted to call the police.

Saatchi said he told the pair that they could pay back the money they owed and live rent-free in a house in Battersea, central London, that Lawson used for filming but his then wife did not agree.

"She was very cross with me," he told jurors. "And she said, 'How could you do that? I would never have them in my house again.' And I said, 'I'm really sorry. I was trying to cause the least friction possible and didn't want a lot of disruption. Sorry if I misread it'."

Saatchi continued: "She then said, 'You have to hand this matter straight over to the police'."

Referring to the atmosphere in the family home before the breakdown of his marriage to Lawson, Saatchi said: "It was a happy home where everyone just buzzed around happily."

He said he was "utterly heartbroken at having lost Nigella" and said he wished this year had never happened.

When asked if he had adored Lawson during their marriage, he said: "I adore Nigella now. I absolutely adore Nigella and I'm broken-hearted to have lost her."

Saatchi said he has no proof of Lawson's alleged drug-taking. Responding to defence barrister Anthony Metzer QC, he said: "Like you, I like proof. I have no proof." He later said: "I have never, never seen any evidence of Nigella taking any drug whatsoever. I want to be helpful, but I genuinely have no real knowledge at all."

When asked by Metzer if he became aware of Lawson's alleged drug-taking around the time of the "Scott's restaurant incident", he said it was indeed around that time.

Metzer asked if he was "shocked", to which Saatchi said he was. "I'm very against drugs," he added.

It is alleged that the Grillo sisters spent the money on designer clothes and handbags from Louis Vuitton, Christian Dior and Vivienne Westwood. They deny fraud.